The present invention relates to a structure of ultrasonic transducer used for ultrasonic diagnosis, more precisely, it relates to a wiring method to a plurality of piezoelectric elements mounted in the transducer head.
Ultrasonic tomography is widely used in diagnosis or failure detection in various materials. In such applications, the transducer head that radiates ultrasonic pulse waves and receives their echoes from various parts of the target is provided with a plurality of piezoelectric elements arranged in an array with a predetermined pitch. The transducer head which is provided with such arrays is called a linear array, phased array or convex array etc., according to the way of arrangement of the piezoelectric elements and the scanning methods of the output wave.
The electronic pulses to energize each of these piezoelectric elements are controlled to shift their phase between each other so as to radiate the ultrasonic wave in a beam directed to a specific direction or focus the beams to a desired point. By controlling the phase of the electronic pulses applied to respective piezoelectric elements, the direction of the output ultrasonic wave beam or its focus can be varied. But these controls can be done only in a plane coplanar with the array. This plane is called the azimuthal plane. The beam scanning is done in the azimuthal direction. In the direction orthogonal to the azimuthal plane the beam can not be scanned, this direction being called the direction of elevation in the art. In the elevation direction, the beam has a fixed expanse determined by the length of each piezoelectric element and the wave length of the output ultrasonic wave.
In order to obtain higher resolution in the azimuthal direction, that is azimuthal resolution of the ultrasonic tomogram, it is necessary to reduce the pitch of the piezoelectric elements in the array, and reduce the size of elements. In a transducer head of recent ultrasonic tomography, 128 piezoelectric elements each 0.55 mm wide and 15 mm long are arranged with a pitch of 0.6 mm, for example. But in order to attain higher resolution or to vary the focal length in the elevation direction, it is necessary to divide the piezoelectric elements in the direction of their length (in the direction of elevation) and arrange the arrays in parallel to each other, so the piezoelectric elements are arranged in a matrix. But there occurs a difficulty of wiring to each of the piezoelectric elements. So, in the state-of-the-art devices only three arrays of the piezoelectric elements are arranged in the elevation direction. But the more fine pitch and the more columns of array in the elevation direction are desirable.
In order to make more apparent the difficulty of the wiring in the ultrasonic transducer head, and to make clear the merits of the present invention, the problem of the wiring to each of the piezoelectric elements will be described. FIG. 1 shows an exemplary transducer head used for ultrasonic diagnosis. In the following explanation, a transducer head for ultrasonic tomography which is used for diagnosis will be referred to as an example, but the explanation can be extended over other applications such as failure detector, or ultrasonic reflectometer etc.
The transducer head shown in FIG. 1, radiates an ultrasonic pulse wave from an acoustic window 21 which passes through freely the ultrasonic wave. The transducer head 20 is contacted with its window 21 to a specimen which is to be tested or to be diagnosed. And the ultrasonic wave is radiated through the acoustic window 21 to the specimen, human body for example (not shown). The reflected waves from various parts of the specimen, such as human organs for example, are detected by the same head 20, converted into electric signals, and transferred to a processor (not shown) by a multi-cored cable 22. In the processor, the detected signals are treated like a manner of radar technology, and provide a tomographic image of the object in the human body.
A unit of the piezoelectric transducer has a structure as shown in FIG. 2(a). A piezoelectric element 1 is sandwiched by electrodes 2A and 2B. By applying electric potential between these electrodes, the piezoelectric element 1 is energized and shrinks or stretches to generate an ultrasonic wave. Contrary, if an echo of the ultrasonic wave reaches the element, an electric potential appears between the electrodes 2A and 2B. In a transducer head, a plurality of such piezoelectric units are arranged in an array, and such arrays are further aligned in parallel to each other to form a matrix as shown in FIG. 2(b). In the figure, three arrays of piezoelectric elements are arranged in a matrix of three columns.
As shown in FIG. 2(b), on the lower surface of the piezoelectric element 1 is provided a front matching layer 10, for matching the acoustic impedance of the piezoelectric element 1 to that of the material which includes the targets in order to transmit the sound energy effectively into the material, human body for example. The words "front" or "back" will be used hereinafter to designate the direction or position referring to the direction toward which the ultrasonic wave is radiated from the piezoelectric element or to its opposite direction respectively. The front matching layer 10 usually has a thickness approximately equal to 1/4 wave length of the ultrasonic wave propagating in the matching layer 10. The front electrodes 2B of these elements are electrically connected to each other and grounded. This connection is usually done by using a conductive material for the front matching layer 10. In front of the front matching layer 10 is provided an acoustic lens (not shown) to focus the ultrasonic wave in the direction of elevation. This acoustic lens is sealed to the case 23 of the transducer head 20, and composes the acoustic window 21.
The matrix of the piezoelectric elements is formed by cutting a large size piezoelectric element in both azimuth and elevation directions by first slits 12 and second slits 13 which are orthogonal to each other. The back electrodes 2A must be connected to respective lead wires. As shown in FIG. 2(b), the piezoelectric elements in arrays on both sides of the matrix can be connected directly to a printed wiring board 11, which has a plurality of contact areas arranged in a position to meet respective piezoelectric elements, and wirings to them are provided on the printed wiring board 11. But it is impossible to attach a printed board directly to the array in the middle column of the matrix. The reason is as follows. On the back side (upper side in the figure) of the piezoelectric elements are provided a backing plate 15 as shown in FIG. 2(c). The backing plate 15 is made of material which absorbs the ultrasonic wave to eliminate a reflection from the back side of the piezoelectric element 1. If there is not provided the backing plate 15, a multi reflection occurs and noise appears in the received signal, which reduces the sensitivity and resolution of the transducer head. Accordingly, if the printed wiring board is connected to the middle column in parallel to the other printed boards 11, it must cross over the other arrays positioned on both sides of the middle column. This causes the reflection. It is sufficient to connect the printed wiring board vertically to the surface of the piezoelectric elements. The difficulty may be easily understood by thinking of the small size of the piezoelectric elements, 0.56 mm wide or less for example.
Therefore in prior art transducer, as shown in FIG. 2, fine bonding wires 14 are bonded to each of the elements in the middle column. Then, the backing plate 15 is formed by molding. After that, another ends of each bonding wires 14 are bonded to respective terminals 17 formed on a terminal plate 16 which is attached on one side of the backing plate 15. In such prior art structure, however, the chance increases to short circuit the bonding wires 14 to each other when the pitch of the array is decreased, especially the short circuits are apt to occur during the molding process of the backing plate 15. The difficulty in bonding the wires to each back electrode 2A rapidly increases as the size and pitch of the unit piezoelectric element is reduced. Further it is difficult to keep each bonding wire 14 in the backing plate 15 straight and vertical to the plane of the matrix without suffering from disconnection. If the bonding wire is bent in the backing plate 15, it causes the reflection.
By the reasons described above, it was difficult to decrease the pitch of the elements and increase the number of the columns in the matrix of the piezoelectric elements, though the requirement has been increased.